1. Field of the Invention
The present invention relates to a method of refining the grains of the cast magnesium alloy without generating dioxin to improve the mechanical properties of the magnesium alloy.
2. Description of the Prior Art
Methods of grain refining magnesium alloy containing aluminum, such as AZ magnesium alloy, include methods that do not require a grain refiner and methods that require a grain refiner.
The former is a superheating method in which the casting melt is prepared by heating the alloy to around 150 to 250xc2x0 C. (1123 to 1173 K) above the melting point, maintaining it at that temperature for 5 to 15 minutes (300 to 900 seconds), and then rapidly cooling it to the casting temperature. The grain refining mechanism is said to be heterogeneous nucleation by an Alxe2x80x94Mnxe2x80x94Fe compound. Because of the high process temperature, the energy costs of the method are high, and there is also the expense involved in preventing oxidation of the melt and in casting ladle checking and maintenance procedures. Thus, the method is beset by problems of economic feasibility and safety.
The latter includes a carbon addition method in which a carbon-containing compound is added to the melt at around 750xc2x0 C. (1023 K). The grain refining mechanism is said to be heterogeneous nucleation by aluminum carbide (Al4C3) produced by carbon in the compound reacting with aluminum in the melt. In commercial processes C7Cl6 is used to be added as a grain refiner, but this is no longer allowed because it produces dioxins (2,3,7,8-tetrachlorodibenzo p-dioxin Cl2(C6H2)O2(C6H2)Cl2).
There is also the ferric chloride method (Elfinal method) in which ferric chloride (FcCl3) is added to a melt at around 760xc2x0 C. (1053 K) and the melt is maintained for 30 to 60 minutes (1800 to 3600 seconds), giving rise to Alxe2x80x94Mnxe2x80x94Fe compound heterogeneous nuclei that are said to produce the grain refinement. It has been reported that in order to obtain a pronounced refinement effect, the Mn content has to be above a certain value. The problem with this method is corrosion produced by a localized battery effect of the Fe and My.
Compared to the superheating process, grain refinement by adding a grain refiner has the merits of a lower process temperature and suitability for large-volume melts. But, it also has the problem that it produces dioxin, generating a need for a refining agent that can be used instead of C2Cl6.
Also, in the case of the ferric chloride method, corrosion resistance is degraded by the battery effect, so there is a need to replace the FeCl3 with a substance that alters the structure of the Alxe2x80x94Mn compound to effect grain refinement of castings without loss of corrosion resistance.
An object of the present invention is to provide a method of grain refining cast magnesium alloy to improve the mechanical properties of the alloy without producing dioxin or degrading the corrosion resistance.
To attain the above object, the present invention provides a method of grain refining cast magnesium alloy comprising adding, as a grain refiner, (i) pure carbon powder or (ii) a carbon source in combination with niobium pentoxide (Nb2O5) or vanadium pentoxide (V2O5) to a magnesium alloy melt containing aluminum and manganese.
By using a grain refiner thus comprised of carbon powder alone or a carbon source in combination with niobium pentoxide or vanadium pentoxide, it is possible to refine, without producing dioxin, the grain diameter of cast materials to 100 xcexcm or smaller compared to grain diameters in the order of 140 to 200 xcexcm when a refiner is not used, also improving the mechanical properties of the cast materials.
Particularly, in the case of addition of a carbon source in combination with niobium pentoxide or vanadium pentoxide, there can be obtained an effect of magnesium alloy refinement and an effect of shaping the Alxe2x80x94Mn compound into spheres.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.